1. Field of the Invention
The present invention relates generally to digital radar landmass simulators and, more particularly, to a system and method for synthesizing overlapping chirp waveforms for digital radar landmass simulators.
2. Statement of the Prior Art
Modern pulse compression radars utilize surface acoustic wave (SAW) filter technology in their exciter/receiver least replaceable units (LRUs) to generate expanded pulse waveforms for transmission. The same or similar SAW filters are then used to compress the received return pulses. This technology enables the radar to substantially improve its signal to noise ratio without sacrificing range resolution. It creates extreme difficulty for simulation and test equipment, however, since the return from multiple discrete targets overlap in time.
The coincident timing of radar returns implies that the simulation equipment must be capable of generating multiple, individually phased waveforms in order to simulate closely spaced targets. This is no simple task, requiring the summation of complex analog waveforms for each independent target.
The problem faced in adapting digital radar landmass simulators (DRLMSs) to pulse compression radars is that the land mass simulation effectively requires a semi-infinite continuum of closely spaced targets. It is not practical to simulate this continuum by processing the expanded waveforms; there are simply too many time-overlapping signals to create. Until recently, there was no option but to embed, into the test equipment, a SAW expansion filter identical to the ones in the radar. This is, in fact, the industry standard approach.
SAW waveform expansion/compression filters are very expensive, however, and each compression mode of each radar is likely to have a different SAW. In order to cover a wide variety of target radars, the existing technology requires an expensive proliferation of dedicated hardware.
A Frequency Agile Signal Generator for Emulating Communications Environments described in the prior art is used to generate simulated communication signals which are inherently different from the simulated radar signals produced in the Chirp/Doppler IF Generator. The communication signals are continuous in nature and come from only one source and are frequency agile. The radar signals are pulse signals and come from many radar reflectors and have a base frequency modulated by the Doppler effect. The central element in the Chirp/Doppler IF Generator, the waveform generator, is used to overlap many complex pulse signals from many radar reflectors.
A prior art Digital Signal Generator teaches a circuit that is the basic accumulator circuit used in commercially available Numerically Controlled Oscillators (NCOs), except that commercial NCOs add a sine or cosine lookup table implemented in a ROM to provide a sine wave rather than the sawtooth wave described in the invention. The technique is known as Direct Digital Synthesis (DDS) and is in wide use in the industry. The Chirp/Doppler IF Generator does use a number of NCOs in its implementation, but the technique used by the NCOs is incidental to the design of the Chirp/Doppler IF Generator. That is, the Chirp/Doppler IF Generator could be designed without NCOs and still function.
A Digitally Controlled Signal Generator uses a recirculating shift register to produce one arbitrary waveform. In contradistinction, the Chirp/Doppler IF Generator uses digital filtering techniques to generate synthetic radar signals that include the effect of many overlapping complex waveforms.